In recent years, individual recognition technology in which individual identification information is given to an individual object to clarify information such as a history of the object has attracted attention. In particular, the development of semiconductor devices which can transmit and receive data without contact by utilizing wireless communication through a radio wave has been advanced. Such semiconductor devices are referred to as an IC tag, an RFID tag, and the like, which have begun to be used for management of articles in markets, or the like.
In general, many of objects which are referred to as an RFID tag and the like that have been put into practical use include an element formation layer and an antenna layer each having a circuit formed of a transistor or the like. Such semiconductor devices capable of wireless communication perform wireless communication with a reader/writer through an electromagnetic wave, so that the semiconductor devices can be operated by receiving power from a power supply and data from the reader/writer. In wireless communication between the reader/writer and the semiconductor device, in general, a transmission side device (a reader/writer) transmits a modulated carrier wave and a reception side device (a semiconductor device) demodulates the carrier wave, so that data is extracted and information is transmitted and received.
In the semiconductor devices capable of wireless communication, there is an amplitude modulation method (an amplitude shift keying (ASK) modulation method) as one of methods which modulate a carrier wave. An ASK modulation method is a method which transmits information by generation of a difference in amplitude of a carrier wave and use of this amplitude difference as a modulation signal. Here, when an amplitude in a low state (a difference between a signal line (an amplitude peak) and a reference line (an amplitude center) is a and an amplitude in a high state (a difference between a signal line (an amplitude peak) and a reference line (an amplitude center) is b, a modulation factor m can be expressed as m=(b−a)/(b+a). The transmission side device can decide a modulation factor by a method in accordance with both the transmission side device and the reception side device which perform wireless communication. The reception side device receives a modulation signal whose modulation factor is decided by the transmission side device, and demodulates the modulation signal.
However, since the semiconductor device capable of wireless communication includes an integrated circuit provided with a minute semiconductor element, a problem of variation or the like of element characteristics arises and it has been difficult to manufacture a semiconductor device having favorable electrical characteristics. Thus, a semiconductor device having favorable electrical characteristics by various improvements has been proposed (for example, see Reference 1).
Note that the semiconductor device capable of wireless communication performs transmission, reception, and the like of a signal in accordance with a communication method based on various standards as communication thereof. With a communication method which is standardized with ISO/IEC 15693 which is a standard of a vicinity-type wireless IC card, a 13.56 MHz carrier wave is modulated in order to obtain a modulation factor of 100% or 10%, and data is encoded with a pulse position modulation method which identifies data by change of a modulation position of a pulse position. There are ISO/IEC 14443 (TYPE-A) and ISO/IEC 18000-3 as standards similar to ISO/IEC 15693. In ISO/IEC 14443 (TYPE-A), it is prescribed that a carrier wave whose modulation factor is 100% is expressed with an amplitude whose initial amplitude (an amplitude in a state without modulation) is less than or equal to 5%.
Thus, various methods are considered in order to demodulate a signal whose modulation factor is 10% (for example, see Reference 2).